The DER triode design rom Marconi dates from 1921. The filament is thoriated tungsten and so emission is achieved at a lower temperature than with pure tungsten, hence the description dull emitter. The construction is of a cylindrical anode with a helical grid passing through it, that in turn is threaded by the filament.

The balloon envelope is 43 mm in diameter and, excluding the B4 pins, is 80 mm tall.

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This exhibit was last updated on 08 July 2009